Article having electrically conductive coating and process of making



Feb. 5, 1963 s. H. HARWIG 3,076,727

ARTICLE HAVING ELECTRICALLY CONDUCTIVE comma AND PROCESS OF MAKING Filed Dec. 24, 1959 P20756777! LAYER 17 c/veolwu/M 16 won 01m 15 z cre/cwaycamm/zwm 5555s; Arie INVENTOR.

ATTORNEYS United States Patent 'Ofifice 3,076,727. Patented Feb. 5, 1963 V 3,676,727 ARTIQLE HAVING ELECTRlQALLY CONBUCTIVE CGATENG AND PRQQESS F MAKING fitephen H. Harwig, Pittsburgh, Pa., assignor to Libbey- Gwens-Ford Glass tlompany, Toledo, Ohio, 21 corporalieu of Ohio Filed Dec. 24, 1959, Ser. No. 861,823 Claims. (Cl. 117-211) This invention relates to electrically conductive films and more particularly to improvements in transparent electrically conductive films and methodsof producing the same.

This application is a continuation-in-part of co-pending application Serial No. 555,162, filed December 23, 1955, now abandoned.

Electrically conductive films are commonly placed on vehicular windows, instrument windows, lenses and other uses so that the surfaces or article may be heated toreduce fogging or icing effects. For many purposes, experience has indicated that the electrical resistivity of the electrically conducting film should not be more than 150 ohms per square and preferably less than 100 ohms per square so as to give'the proper heating effect without excessive voltages. To meet these requirements, it is of course extremely desirable that it be possible to control the resistivity of the film, within the limits, so as to make the film adaptable to difi erent and varied uses.

Additionally, it is desirable thatthe electrically conductive films be hard, durable, and be strongly adherent to glass surfaces. Moreover, when used for optical purposes it is'desirable that the conductive films be highly transparent and free of imperfections and distortions.

It is therefore aprimary object of the invention to provide an electrically conductive film which is hard and durable.

Another objector the invention is to provide a transparent electrically conductive film which has a resistivity of not more than 150 ohms per square and a light transmission of not generally less than 50%.

Other objects and advantages of the invention will become more apparent during the course of the following description when taken in connection with the accompanying drawings.

in the drawings, wherein like numerals are employed to designate like parts throughout the same:

FIG. 1 is a diagrammatic view illustrating'an electrically conductive article having an electrically conductive film of the invention placedthereon; and

FIG. 2 is a sectional view of the article taken along the lines 2-2 of'FlG. 1 showing the respective coatings which comprise the electrically conductive film.

Basically, the present electrically conductive film is an improvement on the electrically conductive film disclosed in Patent No. 2,628,927, issued February 17, 1953, to W. H. Colbert, A. R. Weinrich and W. L. Morgan. As noted therein, the electrically conductive film includes a transparent layer of gold, silver, copper, iron or nickel deposited directly upon and adhered to an adhesive layer formed on a support body. The adhesive layer is preferably a metallic compound such as a metallic oxide, metal sulphide, a metal halide, a metal sulphate, or other metallic compound.

With reference now to the accompanying drawings, there is shown in FIG. 1 an electrically conductive article 10 which comprises a support body 11 of glass; an electrically conductive film 22 on a surface thereof; and electrodes 13 which distribute power to the electrically conductive film. More particularly, the conductive film ineludes, in succession, an adhesive layer 14 in contact with the support 11; -a layer of gold, nickehcopper, silver, or

has been found that the metallic oxides are typical adhesors and adhere by molecular forces to the smooth glass or other siliceous surfaces and also act, by strong molecular adhesion, to hold the metal film. The adhesive metal oxides, for best results, may be those of iron, lead, silver, aluminum, magnesium, nickel, zinc, thorium and other rare earth metallic oxides and the oxides of cadmiilm, antimony, bismuth, mercury, copper, gold, platinum, palladium and other heavy'm-et'al oxides, which are highly adherent to glassy siliceous surfaces and to the metals named above.

The adhesive layerM is preferably very thin being only a few angstroms thick and not visible or otherwise detectable except for the fact that they do permit the forming of the highly adherent articles herein described. The thickness of the layer necessary to develop adhesive forces and to present a surface for forming thereon a continuous metallic layer deposit needs to be only a few angstroms thick and as such, the presence of these on the glass may not be detectable by any optical effects.

These adhesive layers or coatings may be deposited on the support body by direct thermal evaporation or a metallic layer first deposited on the support body by thermal evaporation may be oxidized to form a metallic oxide. A further way in which the layers of metallic oxide may be produced is by first applying a thin coating on the support body by sputtering a metal in a residual vacuum which comprises in part oxygen such that the metal is combined with the oxygen remaining in the air to form an oxide when deposited on the glass.

After the adhesive layer 14 has been placed on the support body 11, the electrically conductive metallic layer 15' is deposited in a thickness of approximately 60 augstroms over the adhesive layer. The conductive layer is deposited by means of thermal evaporation so that an extremely uniform coating may be formed. Even slight variations in the thickness of the electrically conductive layer will result in areas of variable electrical conductivity and thus hot spots or areas of uneven heating characteristics will develop along the thicker portions of the layer.

By using thermal evaporation methods to deposit the electrically conductive layer 15, there is deposited one molecule of metal upon the other in a manner to form a smooth surface over the adhesive layer 14. The adhesive layer on the surface of the support body reduces the possibility of a chemical reaction-or intermingling taking place between the material ot" the support body and the metal of the electrically conductive layer and it will thus be apparent that an extremely effective adhesive layer is provided as a result of the inherent molecular forces of attraction which exist between'the respective materials.

According to thepresent invention, after the electrically conductive layer 15 has been placed on thesupport body in contact with the adhesive layer 14, there is deposited a layer 16 of iron oxide about 2 or 3 angstroms in thickness over the entire'surface of the electrically conductive layer. A layer of chromium 17 of generally 5 to ll'angstroms in thickness is then deposited over the layer of metallic oxide 16 by thermal evaporation. If desired, a protective layer 18 of quartz, aluminum oxide, magnesium fluoride or other suitable material may be then placed over the chromium layer.

Electrodes or bus bars 13 as shown in FIG. 1 may then be placed upon a pair of opposed marginal edge portions of the support body 11 in contact with the electrically conductive film. These electrodes may be of any one of a number of dilferent materials and can be applied before the protective layer 18 is placed over the chromium layer 17, or they may be placed on the support body after the protective layer has been placed over the chromium except for areas corresponding to the electrode locations. As examples of suitable materials for the electrodes or bus bars, those of sprayed, dipped, brushed, or stenciled conductive coatings containing gold, silver, platinum, or copper powders all have been used satisfactorily. Thermally evaporated electrodes of gold, silver, platinum, palladium, chromium, and copper have also been used satisfactorily.

The resistivity characteristics of the electrically conductive iilm 12 are changed by baking the film at elevated temperatures. Specifically, by baking the film at higher temperatures, it has been found that the resistivity of the film progressively decreases, within limits, according to the length of the time baked. It is believed that this change in resistivity is brought about by growth of the crystals of the electrically conductive metal layer at elevated temperatures which causes them to expand and make better electrical contact with one another resulting in increased conductivity, or in other words, decreased resistivity.

The temperature at which the change in resistivity begins to take place has been found to be in the vicinity of 500 F. for the lower limit. However, the temperature must not exceed a temperature at which the crystals tend to form in different planes since this effect apparently causes the crystals to overlap one another thus breaking the electrical continuity in certain areas and reducing or destroying the conductivity of the conductive layer .15. Generally, this overlapping crystallization effect takes place when the film is baked at temperatures generally in excess of 625 F, however, it will be apparent that both the upper and lower limits may vary depending on the type of electrically conductive metal layer 15 being used.

According to the present invention, it has been found that after baking the electrically conductive film 12 is considerably harder and more durable than previous electrically conductive films. This is believed caused by a migration effect which takes place between the chromium layer 17 and the metal conductive layer 15 during the baking process, migration being defined as the afiinity of the molecules of one material to alloy with those of another. Thus, the chromium layer 17 is alloyed with the metal layer 15 through the iron oxide layer 16.

Basically, it is believed that the chromium molecules and the molecules of the metal layer 15 migrate toward one another and interlock thus causing the film to become stronger, tighter and harder. The migration effect is controlled by the iron oxide layer 16 between the respective layers which offers some resistance to the fiow of the molecules toward one another. While the chromium does have some effect upon the resistance characteristics of the film, it should be pointed out that it is not substantial since the amount of chromium which is allowed to migrate and alloy with the metal layer is relatively small as compared to the thickness of the electrically conductive metal layer 15' which is of low resistivity.

It is also believed a migration effect takes place between the chromium layer 17 and the protective layer 18 and thus the chromium layer helps anchor the protective layer. As a result of the migration effect between the chromium and the metal conductive layer 1.5, and the chromium and the protective layer 13, which appears to result in an interlocking of the molecules of the respective materials, a hard crust is formed over the film whic will not mar or deflect easily as was sometimes the case A2. in previous films and thus the film will not readily crack or peel making it harder and more durable.

As another feature of the invention it has been found that the light transmission of tie article generally in creases after baking for periods above 350 F. This is more pronounced at temperatures above 500 F. and generally it has been found that the increase in light transmission ranges from 4 to 10% which is quite substantial and desirable as maximum light transmission is generally important.

For purposes of illustration and to further elaborate upon the features of the invention a number of examples will be given below. It will be appreciated that the electrical resistances mentioned throughout this specification and used in the examples to follow are given as ohms per square area; thus it a film has an electrical resistivity of ohms per square area it has such a resistivity regardless of whether it is one inch square or one foot square.

Example I A substantially square support body was coated, successively, with a layer of iron oxide approximately 5 angstroms in thickness placed in contact with the support body, a gold layer approximately 62 angstroms thick, :1 second layer of iron oxide approximately 5 angstroms thick, a layer of chromium approximately 11.3 angstroms thick, and a layer of quartz approximately a quarter wavelength in thickness.

Bus bars were then placed in contact with the electrically conductive film along areas where the quartz and chromium had been masked from the gold layer. The article so produced after being coated was quite soft and a pencil eraser with mo crate pressure easily marked and removed the electrically conductive film. The resistivity of the conductive iilm between the bus bars was checked and found to be 360 ohms per square. The article was then placed in an oven and baked for three hours at 550 B, after which, it was removed and the resistivity of the electrically conductive film was found to be 29.6 ohms per square. The article was then placed in the oven for another three hour period and baked at 550 B; after this baking it was found to have a resistivity of 27.1 ohms per square. The article was again placed in the oven and baked for three hours at 550 F. and the resistivity was found to be 25.2 ohms per square.

The hardness of the article was then checked and it was -ound that the pencil eraser used previously with increased pressure did not make the slightest mar or scratch on the electrically conductive film.

The article so coated had a light transmission of 63% efore baking and a light transmission of 70% after baking.

Example 11 A vitreous siliceous support substantially square in shape was coated in a manner somewhat similar to that described in Example I except that the chromium layer was approximately 7 angstroms thick. The article was then placed in an oven and baked for two hours at 350 F. The resistivity of the electrically conductive film at this point was found to be 52.8 ohms per square. The article was then baked for two more hours at 550 F. and resistivity was found to be 56.4 ohms per square. After an additional three hour baking at 550 F. the resistivity was 43.3 ohms per square; after another three hour baking at 550 F. the resistivity was 40.0 ohms per square; after still another three hour baking at 550 F. the resistivity was 37.4 ohms per square; and after eight additional hours at 550 F. the resistivity was 34.7 ohms per square.

The coating so produced was extremely hard and durable and the pencil eraser used above in Example I did not mar or scratch the coating. The light transmission of the article was 64% before baking and 71% after baking.

r A substantially'xsquare glass. support body. was coated asin'iExamples l andlll except that the chromium layer was approxim atel-y; angstromsvthick. Before 7 baking the. electrically conductivev .coatingg had a resistivity of 47:0 ohms-per Fsquareand a :light transmissionof 62%. The articleiwaszthenvbaked fortone hour at 550". F. and was found to have anesistivity 01341. 5 ohms per square. Thearticlewasrthen baked -at 6009 F. for two-hours after whichyit Was-foundto have aresistivi-ty of 40.5 ohms per square andia light tnansmission :of .-68-%. The electrically; conductivefilm was extremely hard, and where a-steelitapeteasilyymarred the film before baking. it .did I not manit after baking.

' Example IV An article was producedin'thesamemanner as the article. in Example HI. Before baking'the resistivity was 37.5 ohms'per square and the) light transmission was approximately 164%": Aftereach of four successive 45 minute-bakings at 500 the'respective resistivities were 3415 ohms per*square,' 3l.5 ohms per square, 29.0-ohms per square and 29.0 ohms per square. The light transmission after the four bakings was 72%. The film was very hard-and neithenrubbing with an eraser. as in Example -I nor v dragging .of. the steel tape across the film produced any mars or scratches.

Example V 1 An article was coated'in' a'manner similar to Example I ll. Beforeb'akingtheresistivity was '31 ohms per square with a' light trans-missionof 63% After baking for 45 minutes at 625 F. the resistivity was 30 ohms per square, after a second 45 minute baking period at 570 F. the resistivity was 28 ohms per square, and after a third 45 minute baking period the resistivity was still 28 ohms per square. The light transmission after baking was 70%.

Example VI A glass plate support body was coated as in Example I. The article before baking had a resistivity of 35 ohms per square. It was then baked for 45 minutes at 640 F. and then had a resistivity of 45 ohms per square. After another 45 minute baking period at 640 F. the resistivity was 60 ohms per square and after a third 45 minute baking period at the same temperature the resistivity was 90 ohms per square. The increase in resist-ivity apparently was caused from a crystallization effect which causes individual crystals to form and overlap one another thus breaking the electrical continuity between crystals.

Example VII An article was coated with layers similar to those in Example I and has a resistivity of 33.5 ohms per square before baking with a light transmission of 63%. The article was then baked for 3 hours at 470 F. and had a resistivity of 33.1 ohms per square. After two more successive 3 hour bakings at 470 F. the resistivity remained at 33.0 ohms per square and had a light transmission of 66%. The resistivity therefore did not drop in steps as was the case when the article was baked above approximately 500 F. although there was an increase in light transmission.

Example VIII An article was coated with layers similar to those in Example 'I and had a resistivity of 35.1 ohms per square before baking and a light transmission of 63%. The article was then baked for 4 hours at 350 F. After this baking the resistivity of the film remained substantially at 35 ohms per square, however, the light transmission of the article increased to 66%. After a second baking of 45 minutes the light transmission and resistivity of the article remained substantially the same.

Example IX An article was; produced substantially in the same manner as Example .III.v Before baking the resistivityv was 37.2 ohms per square and the light transmission was approximately .67%. After baking for 2 hours at 350 F. the. light transmission increased to 74% while the resistivityol the article remained the same. Inthe above examples, it will be. readily apparent that the resistivity characteristics of the electrically conductive film -can becontrolle'dby baking the film such that the crystals: of the film grow and make better contact with one another and thus decrease the resistivity; As noted from the examples. the resistivity decreases fro-mapproxi-' mately 2 to 5 'ohms per square torseve'ral baking periods of about 45 minutes, or longer at temperatures above about 500 F. andbelow about 625 F.

Moreover, it willlalso be noted that 'theelectrically conductivefilm has veryexcellent hardness and durability characteristics after it has beensubjected to the bakingtreatment. These features areextremel'y desirable from the standpoint of military. and civilian uses where becausewot abrasive particles in'the air or because of abrasive characteristics of wiping cloths used by maintenance personneL'the electrically conductive films on windshields orinstruments are marred for scratched very easily. Also it is pertinent to note that the light transmission of the articleincreases after baking at the high temperatures which. is generally desirable.

It is tobe understood that the forms of the invention disclosed hereinare' to be taken as the preferred embodiments thereof and that various changes in the shape, size -a.ndarrangement of parts may be resorted to without departing from the spirit of the invention or the scope of the following claims.

I claim:

1. A method of increasing the hardness and durability of an electrically conducting transparent film, composed of a metal selected from the group consisting of gold, silver, copper, iron and nickel, comprising the steps of depositing a transparent layer of iron oxide on the electrically conducting film depositing a transparent layer of chromium on the iron oxide layer, and baking the resulting composite film at a temperature in the range of approximately 350 F. to 625 F. to cause migration between the molecules of the chromium layer and the molecules of the electrically conducting film.

2. A method of increasing the hardness and durability of an electrically conducting film as claimed in claim 1, wherein the iron oxide layer is approximately 2 to 3 Angstroms in thickness and is deposited on the electrically conducting film to control the molecular migration between the chromium layer and the electrically conducting film.

3. A method of increasing the hardness and durability of an electrically conducting film as claimed in claim 1, wherein both the iron oxide layer and the chromium layer are each deposited by thermal evaporation.

4. A method of increasing the hardness and durability of an electrically conducting film as claimed in claim 1, wherein the composite film is baked at a temperature between 500 F. and 625 F.

5. A method of increasing the hardness and durability of an electrically conducting film as claimed in claim 1, wherein a protective layer of a hard metal oxide is deposited on the chromium layer and said baking causes migration between the molecules of the chromium layer and the molecules of the protective layer.

6. A method of increasing the hardness and durability of an electrically conducting film as claimed in claim 1, wherein a protective layer of magnesium fluoride is deposited on the chromium layer and said baking causes migration between the molecules of the chromium layer and the molecules of the protective layer.

7. A method of increasing the hardness and durability of a transparent electrically conducting film of the type in which a transparent layer of a metallic compound which exhibits an adhesive attraction for glass surfaces is deposited on a transparent support body and a transparent electrically conducting layer of a metal selected from the group consisting of gold, silver, copper, iron and nickel is deposited on the adhesive layer, comprising the steps of thermally evaporating a transparent layer of iron oxide on the electrically conducting layer, depositing a transparent layer of chromium on the iron oxide layer, and baldng the resulting composite film at a temperature within the range of approximately 350 F. to 625 F. to cause migration between the molecules of the chromium layer and the molecules of the electrically conducting layer.

8. An electrically conducting article comprising a sup port body having a smooth continuous surface, a transparent layer of a metallic compound which exhibits an adhesive attraction for said support body deposited on the support body, a continuous transparent layer of metal selected from the group consisting of gold, silver, copper, iron and nickel on said intermediate adhesive layer, a transparent layer of iron oxide in contact with said metal layer, and a transparent layer of chromium deposited in contact with said iron oxide layer, said chromium layer being alloyed with said metal layer through said iron oxide layer.

9. An electrically conductive transparent article, comprising a support body of transparent glassy siliceous material having a smooth continuous surface, a continuous transparent adhesi e layer deposited on said smooth continuous surface, said adhesive layer comprising a metallie compound characterized by strong molecular adhesion both to glassy siliceous material and to metals, the adjacent surfaces of said body and adhesive layer being in continuous direct surface to surface contact and defining a smooth continuous interface, a continuous layer of a metal selected from the group consisting of gold, silver, copper, iron and nickel deposited on said adhesive layer and permanently and directly adhered throughout its area to said adhesive layer by molecular forces, said metal layer being substantially uniform in thickness, the thickness of said metal layer being such that it has an electrical resistivity of not more than 150 ohms per square area and the light transmission of the article is at least a continuous transparent iron oxide layer in contact with said metal layer, and a chromium layer deposited in contact with said iron oxide layer, said chromium layer being alloyed with said metal layer through said iron oxide layer.

10. An electrically conductive, transparent article as claimed in claim 9, wherein a protective layer of a material selected from the group consisting of hard metal oxides and magnesium fluoride is deposited over said chromium layer.

References Cited in the file of this patent UNITED STATES PATENTS 2,628,927 Colbert et al. Feb. 17, 1953 2,825,687 Preston et a1. Mar. 10, 1958 2,852,415 Colbert et al Sept. 16, 1958 2,914,428 Ruckelshaus et al. Nov. 24, 1959 

1. A METHOD OF INCREASING THE HARDNESS AND DURABILITY OF AN ELECTRICALLY CONDUCTING TRANSPARENT FILM, COMPOSED OF A METAL SELECTED FROM THE GROUP CONSISTING OF GOLD, SILVER, COPPER, IRON AND NICKEL, COMPRISING THE STEPS OF DEPOSITING A TRANSPARENT LAYER OF IRON OXIDE ON THE ELECTRICALLY CONDUCTING FILM DEPOSITING A TRANSPARENT LAYER OF CHROMIUM ON THE IRON OXIDE LAYER, AND BAKING THE RESULTING COMPOSITE FILM AT A TEMPERATURE IN THE RANGE OF APPROXIMATELY 350* F. TO 625* F. TO CAUSE MIGRATION BETWEEN THE MOLECULES OF THE CHROMIUM LAYER AND THE MOLECULES OF THE ELECTRICALLY CONDUCTING FILM. 